The production of fuel ethanol, or other fuels and chemicals, from lignocellulosic feedstocks provides an attractive alternative to the feedstocks predominantly used to date such as corn starch, sugar cane, and sugar beets. The production of fermentation products from these latter sources cannot increase much further as most of the farmland suitable for the production of these crops is in use. Cellulose is an abundant natural polymer, so there is an enormous untapped potential for its use as a source for fuels and chemicals. Also, lignocellulosic feedstocks to be used for fuel or chemical production are inexpensive as they have limited use. Another advantage of using these feedstocks for fuel or chemical production is that lignin, which is a byproduct of the cellulose conversion process, can be used as a fuel to power the conversion process, thereby avoiding the use of fossil fuels. Several studies have concluded that, when the entire production and consumption cycle is taken into account, the use of ethanol produced from cellulose generates close to nil greenhouse gases.
The conversion of lignocellulosic feedstocks to a fermentation product is usually carried out with a pretreatment process prior to subsequent steps carried out to liberate glucose from the cellulose contained in the feedstock. Pretreatment makes the feedstock more amenable to subsequent conversion of the cellulose to glucose carried out with cellulase enzymes. The glucose can then be converted to a fermentation product such as ethanol by yeast or bacterium using known methods.
Pretreatment of lignocellulosic feedstocks with dilute sulfuric acid at elevated temperature is commonly described in the literature. Examples of other known pretreatment methods that have been proposed include Organosolv™ pretreatment using an organic solvent and alkali pretreatment using ammonia or other alkali. One chemical pretreatment that has received attention in recent years is pretreatment with sulfur dioxide. Sulfur dioxide is a gas, which when dissolved in water, is referred to as sulfurous acid. Sulfur dioxide and/or sulfurous acid can be added to the lignocellulosic feedstock prior to or during a pretreatment by any of a number of methods, including adding sulfur dioxide gas to the lignocellulosic feedstock or combining dilute sulfurous acid to the lignocellulosic feedstock.
However, there are numerous challenges associated with the use of sulfur dioxide and/or sulfurous acid as a pretreatment chemical. In particular, pretreatment with sulfur dioxide and/or sulfurous acid may produce sulfonated species, such as organic compounds in soluble form, known as sulfonic acids and that these organic compounds may be difficult to handle and dispose of. The production of one or more sulfonic acids and the difficulties with their processing and disposal may impede the commercialization of sulfur dioxide and/or sulfurous acid pretreatment.